Willow bark’s (Salix spp.) long history as a pain reliever was reinforced in a study with nearly 200 people who had chronic low-back pain. Participants were able to combine willow with up to 400 mg of the prescription pain reliever tramadol. More than one-third of the participants who took 240 mg (standardized to the active ingredient salicin) and one-fifth of those taking a lower dose were pain-free by the last week of the month-long treatment. This is much higher than the 6 percent of those who ended up without pain in a placebo group.
Source: Chrubasik, S, et al. “Treatment of low back pain exacerbations with willow bark extract: A randomized double-blind study.” American Journal of Medicine 2000, 109: 9–14.